LibELF: Support weak symbols when using BIND_NOW

When using BIND_NOW (e.g. via -Wl,-z,now) we would fail to load ELF
images while doing relocations when we encounter a weak symbol. Instead
we should just patch the PLT entry with a null pointer.

This can be reproduced with:

$ cat test.cpp
int main()
{
    std::cout << "Hello World!" << std::endl;
}
$ g++ -o test -Wl,-z,now test.cpp
$ ./test
did not find symbol while doing relocations for library test: _ITM_RU1

0 files changed, 0 insertions, 0 deletions